1. Field of the Invention
The present invention relates to a differential transmission line, and more particularly to a differential transmission line for transmitting an analog high frequency signal of a microwave range and an extremely high frequency range, or a digital signal.
2. Description of the Related Art
Differential signal transmission is less prone to radiation than the conventionally-used single-ended signal transmission, and is immune to noise. Therefore, differential signal transmission is beginning to be used for high-speed signal transmission. FIG. 21A is an upper plan view showing the transmission line structure of a differential transmission line. FIG. 21B and FIG. 21C are cross-sectional views taken along line A-B in FIG. 21A.
The illustrated structure includes a circuit board 101 and a ground conductor layer 105 which is formed on an inner layer face or a rear face of the circuit board 101 as depicted in FIGS. 21B and 21C. On an inner layer face or a front face of the circuit board 101, two signal conductors 102a and 102b are formed. High frequency signals of opposite signs are supplied to the two signal conductors 102a and 102b, so that they function together as a differential transmission line 102c. 
The signal conductor 102a and the ground conductor Layer 105 compose a first transmission line (microstrip line), whereas the signal conductor 102b and the ground conductor layer 105 compose a second transmission line (microstrip line). The differential transmission line is composed of this pair of transmission lines.
When two microstrip lines are placed adjacent and in parallel to each other and allowed to couple, two modes will occur: an even mode, where signals in the same direction are transmitted through the two microstrip lines; and an odd mode, where signals in opposite directions are transmitted through the two microstrip lines. In a differential transmission line, signals are transmitted by utilizing the odd mode.
FIG. 21B schematically shows directions of electric-field vectors under the odd mode with arrows. FIG. 21C schematically shows directions of electric-field vectors under the even mode with arrows.
Under the odd mode, as shown in FIG. 21B, electric-field vectors generally head from the one signal conductor 102a toward the other signal conductor 102b, while the electric-field vector heading from the signal conductor 102a toward the ground conductor 105 has only a small magnitude. Therefore, in differential transmission under the odd mode, transmission characteristics are not likely to be greatly influenced by any change in the structure of the ground conductor 105. In differential transmission under the odd mode, a virtual ground plane is formed at a symmetric plane between the two signal conductors 102a and 102b. 
On the other hand, the even mode illustrated in FIG. 21C corresponds to an in-phase mode, which is unwanted in differential transmission. Transmission under the in-phase mode suffers from a drastically increased unwanted radiation as compared to transmission under the differential mode (odd mode). Therefore, the in-phase mode must be suppressed. A stronger coupling occurs between the two transmission lines of a differential transmission line pair as the distance between the signal line 102a and the signal line 102b becomes shorter. Therefore, in order to suppress the even mode, it is effective to reduce the gap between the signal line 102a and the signal line 102b. 
However, the fabrication process imposes limits to reducing the gap between the lines, thus making complete suppression of the even mode impossible. Therefore, when designing a differential transmission line, it is imperative to employ a circuit design which prevents any input differential signal from being converted into an in-phase signal. For example, in order for two signals which are input in opposite phases and with an equal amplitude to retain their opposite phases and equal amplitude, it is necessary to maintain circuit symmetry between the two signal lines 102a and 102b, through which the respective signals are transmitted. In other words, the two signal lines 102a and 102b composing the differential transmission line must be two lines which are identical in terms of both amplitude characteristics and phase characteristics.
However, at a curved region of a differential transmission line (i.e., a curving region of the two signal lines 102a and 102b), unwanted mode conversion from a differential signal to an in-phase signal is likely to occur.
Japanese Laid-Open Patent Publication No. 2004-48750 (hereinafter “Patent Document 1”) discloses a method for removing an unwanted in-phase signal which has been superposed on a differential transmission line. With reference to FIG. 22, the construction disclosed in Patent Document 1 will be described.
In the example shown in FIG. 22, a plurality of slots 121 are formed in a ground conductor layer which lies immediately under a differential transmission line 102c. The slots 121 extend in a direction which is orthogonal to a transmission direction 125 of differential signals. By adopting such a construction, the impedance with respect to the in-phase signal is selectively increased, whereby the in-phase signal is reflected.
In transmission under the differential mode, a virtual high frequency ground plane is formed between the two signal conductors 102a and 102b as depicted in FIGS. 21A, 21B and 21C composing the differential transmission line 102c as depicted in FIGS. 21A, 21B and 21C. Therefore, there is little influence on the transmission characteristics resulting from forming the slots 121 in the ground conductor layer 105 as depicted in FIGS. 21B and 21C. Hence, in the differential transmission line described in Patent Document 1, it is possible to reduce the passing intensity of the in-phase signal without unfavorably affecting the transmission characteristics in the differential mode.
Patent Document 1 also discloses a method for removing an in-phase signal in a curved region of a differential transmission line. Specifically, Patent Document 1 describes that, not only when the differential transmission line has a linear shape but also when it has a curved shape, an in-phase signal can be effectively removed by forming a slot 123 in a direction which is orthogonal to a local transmission direction 127 of a signal. On the other hand, “Routing differential I/O signals across split ground planes at the connector for EMI control”, 2000 IEEE International Symposium on Electromagnetic Compatibility, August 2000 vol. 21-25, pp. 325-327 (hereinafter “Non-Patent Document 1”) discloses principles of in-phase mode removal by forming slots in the ground conductor.
However, although the aforementioned conventional technique is able to reduce the intensity of an in-phase signal which passes through the differential transmission line in response to an input in-phase signal, the technique does not concern “unwanted mode conversion intensity”, which represents an in-phase signal which is output in response to an input differential signal.
“Measurement and computer-aided modeling of microstrip discontinuities by an improved resonator method”, 1983 IEEE MTT-S International Microwave Symposium Digest, May 1983, pp. 495-497 (hereinafter “Non-Patent Document 2”) discloses that, in a curved region of a single-ended transmission line, transmission characteristics are improved by removing a corner 129 of a signal conductor 102d, as shown in FIG. 23. Generally speaking, a ground capacitance which is created between a signal conductor and a ground conductor tends to increase in a curved region of a transmission line, as compared to a straight region. Thus, transmission characteristics are improved by reducing the area of the signal conductor 102d in a curved region. This technique is widely used in the present-day high frequency circuit designs. Software for producing a layout view from a circuit diagram, for example, is often configured so as to automatically remove the corner of any curved region of a signal conductor.
“Modeling of radial microstrip bends”, 1990 IEEE MTT-S International Microwave Symposium Digest, May 1990, pp. 1051-1054 (hereinafter “Non-Patent Document 3”) reports the high-frequency characteristics of a circuit structure which exhibits good values as to the transmission characteristics of a curved region of a single-ended transmission line in a high frequency band. While the construction of Non-Patent Document 2 may suffer from reflection of the transmission signal in a high frequency band, the construction of Non-Patent Document 3 improves the high-frequency characteristics by smoothly bending a signal conductor around an assumed center of curvature at a curved region of a transmission line. Such a construction is also commonly used in a high frequency circuit for transmitting signals of an especially high frequency.
A curved region of a differential transmission line shown in FIG. 24A can be realized based on the disclosure of Patent Document 1. The curved region shown in FIG. 24A has a circuit structure that corresponds to the circuit structure of the curved region shown in FIG. 22 from which the slots 123 are removed.
Also, a curved region of a differential transmission line shown in FIGS. 24A and 24B can be realized based on the disclosure of Non-Patent Document 3. In this case, two signal conductors 102a and 102b are disposed in parallel, while being smoothly bent in a curved region around an assumed center of curvature so that they function together as a differential transmission line 102c. 
With the constructions of Patent Document 1 and Non-Patent Document 1, no effects of suppressing the unwanted mode conversion from a differential signal (i.e., odd mode) to an in-phase signal (i.e., even mode) in a curved region can be obtained. In a curved region of a differential transmission line, a more severe unwanted mode conversion occurs with an increased transmission frequency. Therefore, good transmission in the differential mode cannot be realized by merely providing slots in the ground conductor layer.
Moreover, unwanted mode conversion cannot be sufficiently suppressed by applying the structures of Non-Patent Documents 2 and 3, which are proposed for improving the high-frequency characteristics in single-ended signal transmission, to a curved region of a differential transmission line.